Switchable polymer-dispersed liquid crystal optical elements

ABSTRACT

Transmission and reflection type holograms may be formed utilizing a novel polymer-dispersed liquid crystal (PDLC) material and its unique switching characteristics to form optical elements. Applications for these switchable holograms include communications switches and switchable transmission, and reflection red, green, and blue lenses. The PDLC material offers all of the features of holographic photopolymers with the added advantage that the hologram can be switched on and off with the application of an electric field. The material is a mixture of a polymerizable monomer and liquid crystal, along with other ingredients, including a photoinitiator dye. Upon irradiation, the liquid crystal separates as a distinct phase of nanometer-size droplets aligned in periodic channels forming the hologram. The material is called a holographic polymer-dispersed liquid crystal (H-PDLC).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims priority to U.S. patentapplication Ser. No. 11/002,116, filed Dec. 3, 2004 now U.S. Pat. No.7,068,405, which is a divisional of U.S. patent application Ser. No.09/742,397 entitled “SWITCHABLE POLYMER-DISPERSED LIQUID CRYSTAL OPTICALELEMENTS” filed Dec. 22, 2000, now U.S. Pat. No. 6,867,888, which is acontinuation-in-part of and claims priority to U.S. patent applicationSer. No. 09/033,512 entitled “SWITCHABLE VOLUME HOLOGRAM MATERIALS ANDDEVICES” filed Mar. 2, 1998, now U.S. Pat. No. 6,699,407; wherein U.S.patent application Ser. No. 09/033,512 is a continuation of U.S. patentapplication Ser. No. 08/680,292 entitled “SWITCHABLE VOLUME HOLOGRAMMATERIALS AND DEVICES” filed Jul. 12, 1996 and issued as U.S. Pat. No.5,942,157.

This application further claims priority to and incorporates byreference in its entirety provisional application Ser. No. 60/171,478,filed Dec. 22, 1999 entitled “SWITCHABLE POLYMER-DISPERSED LIQUIDCRYSTAL OPTICAL ELEMENTS” and provisional application No. 60/240,771,filed Oct. 17, 2000, also entitled “SWITCHABLE POLYMER-DISPERSED LIQUIDCRYSTAL OPTICAL ELEMENTS.”

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates generally to switchable grating elements for usein telecommunications optical systems. Specifically, embodiments of thepresent invention relate to the use of polymer-dispersed liquid crystalswitchable holographic elements for wavelength selection and deflection.

BRIEF SUMMARY OF THE INVENTION

Summary of the Problem

The current worldwide expenditure for communications infrastructure is$43 billion/year. Approximately $29 billion of this is for telephonenetworks in the US, with the local exchanges accounting for $20 billion,and the long distance carriers accounting for $9 billion. There has beena dramatic expansion in demand for communications capacity due to anexplosion in traffic driven by the Internet, business data transmission,and the transfer of images. By the year 2005, a 100-fold increase intraffic is likely. The infrastructure of long distance carriers andlocal exchanges must be expanded to meet these demands.

Three approaches underway to meet this growing demand include,increasing the number of optical fibers connecting nodes within thenetwork; increasing the bandwidth through wavelength divisionmultiplexing (WDM) using multiple lasers; and/or increasing the datarate capability of a single laser.

The revolution in telecommunications is placing severe demands onnetwork switching capabilities. This includes areas such as add/dropswitches, attenuators, and cross-connect switches for reconfiguring datachannels. Current state-of-the-art technology uses opto-mechanicalswitches that have limitations in speed, power consumption, and lifetime(i.e., mechanical wear). Electro-optic directional couplers can be madewith electro-optic materials (e.g., LiNbO₃). These have no moving partsand switch quite rapidly. However, they are expensive and have high lossand polarization dependent loss. Thermo-optic switches based oninterferometers have relatively poor cross talk. Finally, semiconductoroptical amplifiers can be used as on/off switches. They are quite fast,but they are expensive and difficult to make polarization independent.Future systems will demand the increased capability promised byall-optical switches.

Similarly, electrically switchable transmission gratings have manyapplications for which beams of light must be deflected or holographicimages switched. Among these applications are: fiber optic switches;reprogrammable N×N optical interconnects for optical computing; beamsteering for laser surgery; beam steering for laser radar; holographicimage storage and retrieval; digital zoom optics (switchable holographiclenses); graphic arts and entertainment; and the like.

Summary of the Solution

A hologram is an interference pattern that is recorded on ahigh-resolution recording plate. Two beams formed by a coherent beamfrom a laser, interfere within the recording plate, causing aninterference pattern. This pattern represents object formation. Theobject formation is a function of the light diffracted from the objectto be recorded when the object is placed in the path of one of the twoformation beams. If after processing, the recording plate is viewedcorrectly by monochromatic light, a three-dimensional image of theobject is seen. When forming a holographic grating, there is no object,per se, which is put into the path of one of the beams. Instead, giventhe wave properties of light, when two beams interact, they will form agrating within the recording plate. This grating, as is explained belowcan be formed so as to have any of a variety of characteristics.

Preferred embodiments of the present invention can utilize a novelpolymer-dispersed liquid crystal (PDLC) material and its uniqueswitching characteristics to form optical elements. The PDLC material ofthe present invention offers all of the features of holographicphotopolymers with the added advantage that the hologram can be switchedon and off with the application of an electric field. The material is amixture of a polymerizable monomer and liquid crystal, along with otheringredients, including a photoinitiator dye. Upon irradiation, theliquid crystal separates as a distinct phase of nanometer-size dropletsaligned in periodic channels forming the hologram. The material iscalled a holographic polymer-dispersed liquid crystal (H-PDLC).

Both transmission and reflection type holograms may be formed with thismaterial. The same concepts embodied in this disclosure are viable forother applications, including those for communications switches,switchable transmission, and reflection red, green, and blue lenses.

Preferred embodiments of the present invention can also provide devicesand methods for (a) selecting at least one specific wavelength opticalsignal from a group of other nearly-equal wavelength optical signals and(b) selectively attenuating one or more wavelength optical signals.Certain embodiments incorporate a fiber optic waveguide geometry whichutilizes at least one wavelength selective switch (i.e., short-periodBragg grating), which can be used as a wavelength selective switchableBragg filter for add/drop multiplexers and for switching specific DWDMwavelengths from a multiple wavelength, multi-mode laser. In a furtherembodiment, a long-period Bragg grating is fabricated for use as avariable optical attenuator.

Preferred embodiments of the present invention can also provideintegrated devices which include optical switches integrated with fibersin order to maintain the cylindrical geometry of fiber optics to controloptical loss, where for example, an optical signal cannot be physicallyseparated from the fiber transmission system.

A first embodiment of the present invention describes a wavelengthselective optical element that includes, a first polymer-dispersedliquid crystal switchable holographic component for diffracting awavelength of an incident beam and a second polymer-dispersed liquidcrystal switchable holographic component for diffracting a wavelength ofan incident beam. The first and second polymer-dispersed liquid crystalswitchable holographic components are located in stacked relationshipwith one another and placed in the path of the incident beam.

A second embodiment of the present invention describes an optical systemfor wavelength selection that includes; a coherent beam for inputtingmultiple wavelengths, a first polymer-dispersed liquid crystalswitchable holographic component for diffracting a single wavelength ofan incident beam; and a second polymer-dispersed liquid crystalswitchable holographic component for diffracting a single wavelength ofan incident beam. The first and second polymer-dispersed liquid crystalswitchable holographic components are located in stacked relationshipwith one another and are placed in the path of the incident beam. Anoutput component receives from the first and second polymer-dispersedliquid crystal holographic components at least one of the followinggroup consisting of at least one of the diffracted single wavelengths,all undiffracted multiple wavelengths, and both the diffracted singlewavelengths and the undiffracted multiple wavelengths.

A third embodiment of the present invention describes an opticalconnector that includes, first matrix comprising N×N polymer-dispersedliquid crystal switchable holographic components for deflecting andtransmitting incident radiation and a second matrix comprising N×Noptical components for accepting the deflected and transmitted incidentradiation.

A fourth embodiment of the present invention describes a polarizationdiversity system that includes a first polarizing beam splitter forreceiving an input beam of light and splitting the input beam of lightinto a first beam of light polarized in first direction and a secondbeam of light polarized in a second direction; a first optical pathincluding a first deflector, a half-wave plate, a second deflector, anda mirror, wherein the first optical path receives the first beam oflight polarized in a first direction from the first polarizing beamsplitter and outputs the first beam of light polarized in a seconddirection; a second optical path including a mirror, a third deflector,a half-wave plate, and a fourth deflector, wherein the second opticalpath receives the second beam of light polarized in a second directionfrom the first polarizing beam splitter and outputs the second beam oflight polarized in a first direction; and a second polarizing beamsplitter for receiving the outputted first beam of light polarized in asecond direction from the first optical path and the outputted secondbeam of light polarized in a first direction from the second opticalpath.

A fifth embodiment of the present invention describes an optical switchthat includes a pair of conductive slides at least one of which istransparent, a voltage source electrically contacted to the pair ofconductive slides, at least one spacer for separating the pair ofconductive slides, a layer of polymer-dispersed liquid crystal materiallocated within the confines of the pair of conductive slides and the atleast one spacer, and a switchable holographic grating formed within thelayer of polymer-dispersed liquid crystal material.

A sixth embodiment of the present invention describes a method forforming a switchable holographic waveguide filter that includes etchinga channel into a substrate, filling the channel with a polymerizablematerial, forming at least two sets of electrodes on the substrate, andexposing the polymerizable material to at least two interfering lightbeams in order to form a hologram therein.

A seventh embodiment of the present invention describes a holographicswitch that includes a substrate, a waveguide within the substratehaving a polymer-dispersed liquid crystal holographic layer therein, andat least two sets of electrodes attached to the substrate andelectrically contacting the polymer-dispersed liquid crystal holographiclayer.

An eighth embodiment of the present invention describes a crossbarswitch that includes N×M, polymer-dispersed liquid crystal holographicelements, where N=M and N is equal to at least 2 and further whereineach of the polymer-dispersed liquid crystal holographic elementsdeflects light in a first state and transmits light in a second state,such that light input at any N, is capable of output at any M byalternating between the first and second states of the polymer-dispersedliquid crystal holographic elements.

A ninth embodiment of the present invention describes a nonblockingswitch that includes multiple polymer-dispersed liquid crystalholographic elements arranged into an N input and an M outputconfiguration, wherein each of the elements alternates between either afirst state and a second state or a first state and a third state, andfurther wherein, light incident upon any N input is coupled to any Moutput without blocking the path of any other N input to M outputcoupling.

A tenth embodiment of the present invention describes a method forforming a switchable holographic filter. The method includes: insertinga first end of a first optical fiber into a first end of a capillarytube and inserting a first end of a second optical fiber into a secondend of a capillary tube, leaving a space within the capillary tubebetween the first end of the first optical fiber and the first end ofthe second optical fiber; filling the space within the capillary tubewith a polymerizable material; and exposing the polymerizable materialto radiation, thereby forming a switchable holographic filter within thecapillary tube.

An eleventh embodiment of the present invention describes anelectrically switchable holographic filter. The filter comprises: asubstrate containing an etched groove; a first and second set of fingerelectrodes positioned on the surface of the substrate on either side ofthe etched groove; a capillary tube containing a switchable gratingpositioned within the etched groove; and first and second opticalfibers, wherein the first optical fiber is inserted into one end of thecapillary tube and the second optical fiber is inserted into the otherend of the capillary tube, such that the switchable grating ispositioned between the inserted first and second optical fibers.

A twelfth embodiment of the present invention describes a magneticallyswitchable holographic filter. The filter comprises: a capillary tubecontaining a switchable grating; a coil of wire wrapped around thecapillary tube; and first and second optical fibers. The first opticalfiber is inserted into one end of the capillary tube and the secondoptical fiber is inserted into the other end of the capillary tube, suchthat the switchable grating is positioned between the inserted first andsecond optical fibers.

A thirteenth embodiment of the present invention describes a thermallyswitchable holographic filter. The filter comprises a capillary tubecontaining a switchable grating positioned between at least two heatingelements and first and second optical fibers. The first optical fiber isinserted into one end of the capillary tube and the second optical fiberis inserted into the other end of the capillary tube, such that theswitchable grating is positioned between the inserted first and secondoptical fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a conventional switchable grating;

FIGS. 2 a and 2 b are elevational views of a reflection grating inaccordance with the present invention having planes of polymer channelsand PDLC channels disposed parallel to the front surface in the absenceof a field (FIG. 2 a) and with an electric field applied (FIG. 2 b)wherein the liquid crystal utilized in the formation of the grating hasa positive dielectric anisotropy;

FIGS. 3 a and 3 b are elevational views of a reflection grating inaccordance with the invention having planes of polymer channels and PDLCchannels disposed parallel to the front surface of the grating in theabsence of an electric field (FIG. 3 a) and with an electric fieldapplied (FIG. 3 b) wherein the liquid crystal utilized in the formationof the grating has a negative dielectric anisotropy;

FIG. 4 is a cross-sectional view of an electrically switchable hologrammade of an exposed polymer-dispersed liquid crystal material accordingto the teachings of the present invention;

FIG. 5 is a schematic view of a recording system for forming atransmission hologram according to the present invention;

FIG. 6 models diffraction of p-polarized light in a PDLC transmissionhologram according to an embodiment of the present invention.

FIG. 7 is a schematic view of a random azimuthal distribution ofsymmetry axes in liquid crystal domains in a PDLC transmission hologramaccording to an embodiment of the present invention.

FIG. 8 is a schematic view of a reorientation of a liquid crystal domainsymmetry axis in the presence of a strong electric field.

FIG. 9 a is an elevational view of a reflection grating in accordancewith the invention disposed within a magnetic field generated byHelmholtz coils;

FIGS. 9 b and 9 c are elevational views of the reflection grating ofFIG. 9 a in the absence of an electric field (FIG. 9 b) and with anelectric field applied (FIG. 9 c);

FIGS. 10 a and 10 b are representative side views of a slantedtransmission grating (FIG. 10 a) and a slanted reflection grating (FIG.10 b) showing the orientation of the grating vector K of the planes ofpolymer channels and PDLC channels;

FIG. 11 is a graph of the normalized net transmittance and normalizednet diffraction efficiency of a hologram made according to the teachingsof the present invention (without the addition of a surfactant) versusthe rms voltage applied across the hologram;

FIG. 12 is a graph of both the threshold and complete switching rmsvoltages needed for switching a hologram made according to the teachingsof the present invention to minimum diffraction efficiency versus thefrequency of the rms (root mean square) voltage;

FIG. 13 is a graph of the normalized diffraction efficiency as afunction of the applied electric field for a PDLC material formed inaccordance with an embodiment of the present invention;

FIG. 14 is a graph showing the switching response time data for thediffracted beam in the surfactant-containing PDLC material;

FIG. 15 is a graph of the normalized net transmittance and thenormalized net diffraction efficiency of a hologram made according tothe teachings of the present invention versus temperature;

FIG. 16 is an elevational view of a subwavelength grating in accordancewith the present invention having planes of polymer channels and PDLCchannels disposed perpendicular to the front surface of the grating;

FIG. 17 a is an elevational view of a switchable subwavelength gratingin accordance with the present invention wherein the subwavelengthgrating functions as a half wave plate whereby the polarization of theincident radiation is rotated by 90 degrees.;

FIG. 17 b is an elevational view of the switchable half wave plate shownin FIG. 17 a disposed between crossed polarizers through which theincident light is transmitted;

FIGS. 17 c and 17 d are side views of the switchable half wave plate andcrossed polarizers shown in FIG. 17 b showing the effect of theapplication of a voltage to the plate through which the polarization ofthe light is no longer rotated and is thus blocked by the secondpolarizer;

FIG. 18 a is a side view of a switchable subwavelength grating inaccordance with the invention wherein the subwavelength gratingfunctions as a quarter wave plate so that plane polarized light istransmitted through the subwavelength grating, retroreflected by amirror and reflected by the beam splitter;

FIG. 18 b is a side view of the switchable subwavelength grating of FIG.18 a showing the effect of the application of a voltage to the plate sothat the polarization of the light is no longer modified, therebypermitting the reflected light to pass through the beam splitter;

FIGS. 19 a and 19 b are elevational views of a subwavelength grating inaccordance with the present invention having planes of polymer channelsand PDLC channels disposed perpendicular to the front face of thegrating, respectively, in the absence of an electric field and with anelectric field applied, wherein the liquid crystal utilized in formationof the grating has a positive dielectric anisotropy;

FIG. 20 is a side view of five subwavelength gratings wherein thegratings are stacked and connected electrically in parallel therebyreducing the switching voltage of the subwavelength grating;

FIG. 21 is a schematic illustration of a remotely configurable opticaladd/drop multiplexer (OADM) according to an embodiment of the presentinvention;

FIGS. 22( a)–(d) are schematic illustrations of a switchable Braggfilter in a channel waveguide during the formation thereof according toan embodiment of the present invention;

FIG. 23 is a schematic illustration of a random distribution of symmetryaxes clustered about K as seen by light propagating down a waveguidechannel according to an embodiment of the present invention;

FIG. 24 is an electrode configuration for switching a PDLC holographicreflection grating according to an embodiment of the present invention;

FIGS. 25( a)–(b) are schematic illustrations of alignment scenarios ofliquid crystal domain axes in a PDLC holographic reflection gratingaccording to an embodiment of the present invention;

FIG. 26 is a schematic representation of a dense wavelength divisionmultiplexing (DWDM) switch according to an embodiment of the presentinvention;

FIGS. 27( a)–(b) are schematic representations of a filter withelectrodes during formation thereof according to an embodiment of thepresent invention;

FIGS. 28( a)–(b) are schematic representations of recording scenariosfor recording long-period gratings according to an embodiment of thepresent invention;

FIG. 29 is a schematic representation of a concatenation of long-periodgratings according to an embodiment of the present invention;

FIGS. 30( a)–(b) are graphs showing distributions of voltage and indexmodulation for a voltage-controlled long-period PDLC grating accordingto an embodiment of the present invention;

FIGS. 31( a)–(b) are schematic representations of a filter integratedwith a fiber during formation thereof according to an embodiment of thepresent invention;

FIGS. 31( c)–(d) are schematic representations of the electrodeconfiguration for switching a filter according to an embodiment of thepresent invention;

FIG. 32 is a schematic representation of a system for switching a filteraccording to an embodiment of the present invention;

FIG. 33 is a schematic representation of a system for switching a filteraccording to an embodiment of the present invention;

FIGS. 34( a)–(b) are schematic illustrations of variable frequency lasersource configurations in accordance with embodiments of the presentinvention;

FIG. 35 is a schematic representation of a beam deflector in accordancewith an embodiment of the present invention;

FIG. 36 is a schematic illustration of a polarization diversity schemefor a matrix cross-connect switch employing PDLC holographictransmission gratings according to an embodiment of the presentinvention;

FIG. 37 is a schematic representation of a 4×4 crossbar switch accordingto an embodiment of the present invention;

FIGS. 38( a)–(b) are schematic representations of individual PDLCswitches for use in a switch architecture according to an embodiment ofthe present invention.

FIG. 39 is a schematic representation of a 4×4 switch utilizing theswitches of FIGS. 38( a)-(b) according to an embodiment of the presentinvention;

FIG. 40 is a schematic representation of an individual PDLC switch foruse in a switch architecture according to an embodiment of the presentinvention;

FIG. 41 is a schematic representation of a 4×4 switch utilizing theswitches of FIG. 37 according to an embodiment of the present invention;

FIG. 42 is a schematic representation of a 4×4 optical cross-connectswitch in accordance with an embodiment of the present invention;

FIG. 43 is a schematic representation of a 9×9 optical cross-connectswitch in accordance with an embodiment of the present invention;

FIG. 44 is a schematic representation of a 16×16 optical cross-connectswitch in accordance with an embodiment of the present invention; and

FIG. 45 is an output efficiency graph for the 4×4 optical cross-connectswitch according to FIG. 42.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A basic component of the optical elements described in detail withinthis disclosure is the polymer-dispersed liquid crystal (“PDLC”)material used therein. Accordingly, a general description of theingredients which comprise this PDLC material as well as variousspecific examples of combinations of these ingredients which are used toform specific types of PDLC materials, are discussed immediately, below.

In accordance with embodiments of the present invention there isprovided a polymer-dispersed liquid crystal (“PDLC”) material made froma monomer, a dispersed liquid crystal, a cross-linking monomer, acoinitiator and a photoinitiator dye. These PDLC materials exhibit clearand orderly separation of the liquid crystal and cured polymer, wherebythe PDLC material advantageously provides high quality holographicgratings. The PDLC materials of the present invention are alsoadvantageously formed in a single step. The present invention alsoutilizes a unique photopolymerizable prepolymer material that permits insitu control over characteristics of the resulting gratings, such asdomain size, shape, density, ordering, and the like. Furthermore,methods and materials of the present invention can be used to preparePDLC materials that function as switchable transmission or reflectiongratings.

Polymer-dispersed liquid crystal materials, methods, and devicescontemplated for use in the practice of the present invention are alsodescribed in R. L. Sutherland et al., “Bragg Gratings in an AcrylatePolymer Consisting of Periodic Polymer-Dispersed Liquid-Crystal Planes,”Chemistry of Materials, No. 5, pp. 1533–1538 (1993); in R. L. Sutherlandet al., “Electrically switchable volume gratings in polymer-dispersedliquid crystals,” Applied Physics Letters, Vol. 64, No. 9, pp. 1074–1076(1984); and T. J. Bunning et al., “The Morphology and Performance ofHolographic Transmission Gratings Recorded in Polymer-Dispersed LiquidCrystals,” Polymer, Vol. 36, No. 14, pp. 2699–2708 (1995), all of whichare fully incorporated by reference into this specification.

A preferred polymer-dispersed liquid crystal (“PDLC”) material employedin the practice of the present invention creates a switchable hologramin a single step. A new feature of a preferred PDLC material is thatillumination by an inhomogeneous, coherent light pattern initiates apatterned, anisotropic diffusion (or counter-diffusion) of polymerizablemonomer and second phase material, particularly liquid crystal (“LC”)for this application. Thus, alternating well-defined channels of secondphase-rich material, separated by well-defined channels of nearly purepolymer, are produced in a single-step process.

A resulting preferred PDLC material has an anisotropic spatialdistribution of phase-separated LC droplets within the photochemicallycured polymer matrix. Conventional PDLC materials made by a single-stepprocess can achieve at best only regions of larger LC bubbles andsmaller LC bubbles in a polymer matrix. This is due to multipleconstraints such as material limitations, exposure times, and sources ofexposure, all of which are well known to those skilled in the art. Thelarge bubble sizes are highly scattering which produces a hazyappearance and multiple order diffractions, in contrast to thewell-defined first order diffraction and zero order diffraction madepossible by the small LC bubbles of a preferred PDLC material inwell-defined channels of LC-rich material. Reasonably well-definedalternately LC-rich channels and nearly pure polymer channels in a PDLCmaterial are possible by multi-step processes, but such processes do notachieve the precise morphology control over LC droplet size anddistribution of sizes and widths of the polymer and LC-rich channelsmade possible by a preferred PDLC material.

The features of the PDLC material are influenced by the components usedin the preparation of the homogeneous starting mixture and, to a lesserextent, by the intensity of the incident light pattern. In a preferredembodiment, the prepolymer material comprises a mixture of aphotopolymerizable monomer, a second phase material, a photo initiatordye, a coinitiator, a chain extender (or cross-linker), and, optionally,a surfactant.

In a preferred embodiment, the two major components of the prepolymermixture are the polymerizable monomer and the second phase material,which are preferably completely miscible. Highly functionalized monomersare preferred because they form densely cross-linked networks whichshrink to some extent and tend to squeeze out the second phase material.As a result, the second phase material is moved anisotropically out ofthe polymer region and, thereby, separated into well-definedpolymer-poor, second phase-rich regions or domains. Highlyfunctionalized monomers are also preferred because the extensivecross-linking associated with such monomers yields fast kinetics,allowing the hologram to form relatively quickly, whereby the secondphase material will exist in domains of less than approximately 0.1 μm.

Highly functionalized monomers, however, are relatively viscous. As aresult, these monomers do not tend to mix well with other materials, andthey are difficult to spread into thin films. Accordingly, it ispreferable to utilize a mixture of penta-acrylates in combination withdi-, tri-, and/or tetra-acrylates in order to optimize both thefunctionality and viscosity of the prepolymer material. Suitableacrylates, such as triethyleneglycol diacrylate, trimethylolpropanetriacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate,pentaerythritol pentaacrylate, and the like can be used in accordancewith the present invention. In a preferred embodiment, it has been foundthat an approximately 1:4 mixture of tri- to penta-acrylate facilitateshomogeneous mixing while providing a favorable mixture for forming 10–20μm films on the optical plates.

The second phase material of choice for use in the practice of thepresent invention is a liquid crystal. This also allows anelectro-optical response for the resulting hologram. The concentrationof LC employed should be large enough to allow a significant phaseseparation to occur in the cured sample, but not so large as to make thesample opaque or very hazy. Below about 20% by weight very little phaseseparation occurs and diffraction efficiencies are low. Above about 35%by weight, the sample becomes highly scattering, reducing bothdiffraction efficiency and transmission. Samples fabricated withapproximately 25% by weight typically yield good diffraction efficiencyand optical clarity. In prepolymer mixtures utilizing a surfactant, theconcentration of LC may be increased to 35% by weight without loss inoptical performance by adjusting the quantity of surfactant. Suitableliquid crystals contemplated for use in the practice of the presentinvention include the mixture of cyanobiphenyls marketed as E7 by Merck,4′-n-pentyl-4-cyanobiphenyl, 4′-n-heptyl-4-cyanobiphenyl,4′-octaoxy-4-cyanobiphenyl, 4′-pentyl-4-cyanoterphenyl,methoxybenzylidene-4′-butylaniline, and the like. Other second phasecomponents are also possible.

A preferred polymer-dispersed liquid crystal material employed in thepractice of the present invention is formed from a prepolymer materialthat is a homogeneous mixture of a polymerizable monomer comprisingdipentaerythritol hydroxypentaacrylate (available, for example, fromPolysciences, Inc., Warrington, Pa.), approximately 10–40 wt % of theliquid crystal E7 (which is a mixture of cyanobiphenyls marketed as E7by Merck and also available from BDH Chemicals, Ltd., London, England),the chain-extending monomer N-vinylpyrrolidone (“NVP”) (available fromthe Aldrich Chemical Company, Milwaukee, Wis.), coinitiatorN-phenylglycine (“NPG”) (also available from the Aldrich ChemicalCompany, Milwaukee, Wis.), and the photoinitiator dye rose bengal ester(2,4,5,7-tetraiodo-3′,4′,5′,6′-tetrachlorofluoroescein-6-acetate ester)marketed as RBAX by Spectragraph, Ltd., Maumee, Ohio). Rose bengal isalso available as rose bengal sodium salt (which must be esterfied forsolubility) from the Aldrich Chemical Company. This system has a veryfast curing speed which results in the formation of small liquid crystalmicro-droplets.

The mixture of liquid crystal and prepolymer material are homogenized toa viscous solution by suitable means (e.g., ultrasonification) andspread between indium-tin-oxide (“ITO”) coated glass slides with spacersof nominally 15–100 μm thickness and, preferably, 10–20 μm thickness.The ITO is electrically conductive and serves as an opticallytransparent electrode. Preparation, mixing and transfer of theprepolymer material onto the glass slides are preferably done in thedark as the mixture is extremely sensitive to light.

The sensitivity of the prepolymer materials to light intensity isdependent on the photoinitiator dye and its concentration. A higher dyeconcentration leads to a higher sensitivity. In most cases, however, thesolubility of the photoinitiator dye limits the concentration of the dyeand, thus, the sensitivity of the prepolymer material. Nevertheless, ithas been found that for most general applications photoinitiator dyeconcentrations in the range of 0.2–0.4% by weight are sufficient toachieve desirable sensitivities and allow for a complete bleaching ofthe dye in the recording process, resulting in colorless final samples.Photoinitiator dyes that are useful in generating PDLC materials inaccordance with the present invention are rose bengal ester(2,4,5,7-tetraiodo-3′,4′,5′,6′-tetrachlorofluroescein-6-acetate ester);rose bengal sodium salt; eosin; eosin sodium salt; 4,5-diiodosuccinylfluorescein; camphorquinone; methylene blue, and the like. These dyesallow a sensitivity to recording wavelengths across the visible spectrumfrom nominally 400 nm to 700 nm. Suitable near-infrared dyes, such ascationic cyanine dyes with trialkylborate anions having absorption from600–900 nm, as well as merocyanine dyes derived from spiropyran shouldalso find utility in connection with the present invention.

The coinitiator employed in the practice of the present inventioncontrols the rate of curing in the free radical polymerization reactionof the prepolymer material. Optimum phase separation and, thus, optimumdiffraction efficiency in the resulting PDLC material, are a function ofcuring rate. It has been found that favorable results can be achievedutilizing coinitiator in the range of 2–3% by weight. Suitablecoinitiators include: N-phenylglycine; triethylene amine;triethanolamine; N,N-dimethyl-2,6-diisopropyl aniline; and the like.

Other suitable dyes and dye coinitiator combinations that should besuitable for use in the present invention, particularly for visiblelight, include: eosin and triethanolamine; camphorquinone andN-phenylglycine; fluorescein and triethanolamine; methylene blue andtriethanolamine or N-phenylglycine; erythrosin B and triethanolamine;indolinocarbocyanine and triphenyl borate; iodobenzospiropyran andtriethylamine; and the like.

The chain extender (or cross-linker) employed in the practice of thepresent invention helps to increase the solubility of the components inthe prepolymer material as well as increase the speed of polymerization.The chain extender is preferably a smaller vinyl monomer as comparedwith the pentaacrylate, whereby it can react with the acrylate positionsin the pentaacrylate monomer, which are not easily accessible toneighboring pentaacrylate monomers due to steric hindrance. Thus,reaction of the chain extender monomer with the polymer increases thepropagation length of the growing polymer and results in high molecularweights. It has been found that chain extender in general applicationsin the range of 10–18% by weight maximizes the performance in terms ofdiffraction efficiency. In a preferred embodiment, it is expected thatsuitable chain extenders can be selected from the following: N-vinylpyrrolidone; N-vinyl pyridine; acrylonitrile; N vinyl carbazole; and thelike.

It has been found that the addition of a surfactant material, namely,octanoic acid, in the prepolymer material lowers the switching voltageand also improves the diffraction efficiency. In particular, theswitching voltage for PDLC materials containing a surfactant aresignificantly lower than those of a PDLC material made without thesurfactant. Scanning electron microscopy (“SEM”) studies have shown thatdroplet sizes in PDLC materials including surfactants are reduced to therange of 30–50 nm and the distribution is more homogeneous. Randomscattering in such materials is reduced due to the dominance of smallerdroplets, thereby increasing the diffraction efficiency. Thus, it isbelieved that the shape of the droplets becomes more spherical in thepresence of surfactant, thereby contributing to the decrease inswitching voltage.

For more general applications, it has been found that samples with aslow as 5% by weight of surfactant exhibit a significant reduction inswitching voltage. It has also been found that, when optimizing for lowswitching voltages, the concentration of surfactant may vary up to about10% by weight (mostly dependent on LC concentration) after which thereis a large decrease in diffraction efficiency, as well as an increase inswitching voltage (possibly due to a reduction in total phase separationof LC). Suitable surfactants include: octanoic acid; heptanoic acid;hexanoic acid; dodecanoic acid; decanoic acid; and the like.

In samples utilizing octanoic acid as the surfactant, it has beenobserved that the conductivity of the sample is high, presumably owingto the presence of the free carboxyl (COOH) group in the octanoic acid.As a result, the sample increases in temperature when a high frequency(˜2 KHz) electrical field is applied for prolonged periods of time.Thus, it is desirable to reduce the high conductivity introduced by thesurfactant, without sacrificing the high diffraction efficiency and thelow switching voltages. It has been found that suitable electricallyswitchable gratings can be formed from a polymerizable monomer, vinylneononanoate (“VN”) C₈H₁₇CO₂ CH═CH₂, commercially available from theAldrich Chemical Co. in Milwaukee, Wis. Favorable results have also beenobtained where the chain extender N-vinyl pyrrolidone (“NVP”) and thesurfactant octanoic acid are replaced by 6.5% by weight VN. VN also actsas a chain extender due to the presence of the reactive acrylate monomergroup. In these variations, high optical quality samples were obtainedwith about 70% diffraction efficiency, and the resulting gratings couldbe electrically switched by an applied field of 6 V/μm.

PDLC materials in accordance with the present invention may also beformed using a liquid crystalline bifunctional acrylate as the monomer(“LC monomer”). The LC monomers have an advantage over conventionalacrylate monomers due to their high compatibility with the low molecularweight nematic LC materials, thereby facilitating formation of highconcentrations of low molecular weight LC and yielding a sample withhigh optical quality. The presence of higher concentrations of lowmolecular weight LCs in the PDLC material greatly lowers the switchingvoltages (e.g., to ˜2 V/μm). Another advantage of using LC monomers isthat it is possible to apply low AC or DC fields while recordingholograms to pre-align the host LC monomers and low molecular weight LCso that a desired orientation and configuration of the nematic directorscan be obtained in the LC droplets. The chemical formulae of severalsuitable LC monomers are as follows:CH₂═CH—COO—(CH₂)₆O—C₆H₅—C₆H₅—COO—CH═CH₂  I.CH—(CH₂)₈—COO—C₆H₅—COO—(CH₂)₈—CH═CH₂  II.H(CF₂)₁₀CH₂O—CH₂—C(═CH₂)—COO—(CH₂CH₂O)₃CH₂CH₂O—COO—CH₂—C(═CH₂)—CH₂(CF₂)₁₀H  III.

Semifluorinated polymers are known to show weaker anchoring propertiesand also significantly reduced switching fields. Thus, semifluorinatedacrylate monomers which are bifunctional and liquid crystalline shouldfind suitable application in the present invention.

In a preferred embodiment, the prepolymer material utilized to make areflection grating comprises a monomer, a liquid crystal, across-linking monomer, a coinitiator, and a photoinitiator dye. In apreferred embodiment, the reflection grating is formed from prepolymermaterial comprising by total weight of the monomer dipentaerythritolhydroxypentaacrylate (“DPHA”), 34% by total weight of a liquid crystalcomprising a mixture of cyano biphenyls (known commercially as “E7”),10% by total weight of a cross-linking monomer comprising N-vinylpyrrolidone (“NVP”), 2.5% by weight of the coinitiator N-phenylglycine(“NPG”), and 10⁻⁵ to 10⁻⁶ gram moles of a photoinitiator dye comprisingrose bengal ester. Further, as with transmission gratings, the additionof surfactants should facilitate the same advantageous propertiesdiscussed above in connection with transmission gratings. Similar rangesand variation of prepolymer starting materials should find readyapplication in the formation of suitable reflection gratings.

It has been determined by low voltage, high resolution scanning electronmicroscopy (“LVHRSEM”) that the resulting material comprises a finegrating with a periodicity of 165 nm with the grating vectorperpendicular to the plane of the surface. Thus, as shown schematicallyin FIG. 2 a, grating 130 includes periodic planes of polymer channels130 a and PDLC channels 130 b which run parallel to the front surface134. The grating spacing associated with these periodic planes remainsrelatively constant throughout the full thickness of the sample from theair/film to the film/substrate interface.

Although interference is used to prepare both transmission andreflection gratings, the morphology of the reflection grating differssignificantly. In particular, it has been determined that, unliketransmission gratings with similar liquid crystal concentrations, verylittle coalescence of individual droplets was evident. Furthermore, thedroplets that were present in the material were significantly smaller,having diameters between 50 and 100 nm. Furthermore, unlike transmissiongratings where the liquid crystal-rich regions typically comprise lessthan 40%, of the grating, the liquid crystal-rich component of areflection grating is significantly larger. Due to the much smallerperiodicity associated with reflection gratings, i.e., a narrowergrating spacing (˜0.2 microns), the time difference between completionof curing in high intensity versus low intensity regions should be muchsmaller. Thus, gelation occurs more quickly and droplet growth isminimized. It is also believed that the fast polymerization, asevidenced by small droplet diameters, traps a significant percentage ofthe liquid crystal in the matrix during gelation and precludes anysubstantial growth of large droplets or diffusion of small droplets intolarger domains.

Analysis of the reflection notch in the absorbance spectrum supports theconclusion that a periodic refractive index modulation is disposedthrough the thickness of the film. In PDLC materials that are formedwith the 488 nm line of an argon ion laser, the reflection notchtypically has a reflection wavelength at approximately 472 nm for normalincidence and a relatively narrow bandwidth. The small differencebetween the writing wavelength and the reflection wavelength(approximately 5%) indicates that shrinkage of the film is not asignificant problem. Moreover, it has been found that the performance ofsuch gratings is stable over periods of many months.

In addition to the materials utilized in a preferred embodimentdescribed above, suitable PDLC materials could be prepared utilizingmonomers such as triethyleneglycol diacrylate,trimethylolpropanetriacrylate, pentaerythritol triacrylate,pentaerythritol tetraacrylate, pentaerythritol pentaacrylate, and thelike. Similarly, other coinitiators such as triethylamine,triethanolamine, N,N-dimethyl-2,6-diisopropylaniline, and the like couldbe used instead of N-phenylglycine. Where it is desirable to use the 458nm, 476 nm, 488 nm or 514 nm lines of an Argon ion laser, that thephotoinitiator dyes rose bengal, rose bengal sodium salt, eosin, eosinsodium salt, fluorescein sodium salt and the like will give favorableresults. Where the 633 nm line is utilized, methylene blue will findready application. Finally, it is believed that other liquid crystals,such as 4′-pentyl-4-cyanobiphenyl or 4′-heptyl-4-cyanobiphenyl, can beutilized in accordance with the invention.

Referring again to FIG. 2 a, there is shown an elevational view of areflection grating 130 in accordance with the invention having periodicplanes of polymer channels 130 a and PDLC channels 130 b disposedparallel to the front surface 134 of the grating 130. The symmetry axis136 of the liquid crystal domains is formed in a direction perpendicularto the periodic channels 130 a and 130 b of the grating 130 andperpendicular to the front surface 134 of the grating 130. Thus, when anelectric field E is applied, as shown in FIG. 2 b, the symmetry axis 136is already in a low energy state in alignment with the field E and willnot reorient. Thus, reflection gratings formed in accordance with theprocedure described above will not normally be switchable.

In general, a reflection grating tends to reflect a narrow wavelengthband, such that the grating can be used as a reflection filter. In apreferred embodiment, however, the reflection grating is formed so thatit will be switchable. In accordance with the present invention,switchable reflection gratings can be made utilizing negative dielectricanisotropy LCs (or LCs with a low cross-over frequency), an appliedmagnetic field, an applied shear stress field, or slanted gratings.

It is known that liquid crystals having a negative dielectric anisotropy(Δ∈) will rotate in a direction perpendicular to an applied field. Asshown in FIG. 3 a, the symmetry axis 136 of the liquid crystal domainsformed with a liquid crystal having a negative Δ∈ will also be disposedin a direction perpendicular to the periodic channels 130 a and 130 b ofthe grating 130 and to the front surface 134 of the grating. However,when an electric field E is applied across such gratings, as shown inFIG. 3 b, the symmetry axis of the negative Δ∈ liquid crystal willdistort and reorient in a direction perpendicular to the field E, whichis perpendicular to the film and the periodic planes of the grating. Asa result, the reflection grating can be switched between a state whereit is reflective and a state where it is transmissive. The followingnegative Δ∈ liquid crystals and others are expected to find readyapplication in the methods and devices of the present invention:

Liquid crystals can be found in nature (or synthesized) with eitherpositive or negative Δ∈. Thus, in some specific embodiments of thepresent invention, it is possible to use a LC which has a positive Δ∈ atlow frequencies, but becomes negative at high frequencies. The frequency(of the applied voltage) at which Δ∈ changes sign is called thecross-over frequency. The cross-over frequency will vary with LCcomposition, and typical values range from 1–10 kHz. Thus, by operatingat the proper frequency, the reflection grating may be switched. Inaccordance with embodiments of the present invention, it is expectedthat low crossover frequency materials can be prepared from acombination of positive and negative dielectric anisotropy liquidcrystals. By way of example, a preferred positive dielectric liquidcrystal for use in such a combination contains four ring esters as shownbelow:

A strongly negative dielectric liquid crystal suitable for use in such acombination is made up of pyridazines as shown below:

Both liquid crystal materials are available from, for example, LaRoche &Co., Switzerland. By varying the proportion of the positive and negativeliquid crystals in the combination, crossover frequencies from 1.4–2.3kHz are obtained at room temperature. Another combination suitable foruse in the present embodiment is a combination of the following:p-pentylphenyl-2-chloro-4-(p-pentylbenzoyloxy) benzoateand-4-(p-pentylbenzoyloxy) benzoate andp-heptylphenyl-2-chloro-4-(p-octylbenzoyloxy) benzoate. These materialsare available from Kodak® Company. A preferred embodiment of the presentinvention comprises a basic optical grating component formed with thepreviously described PDLC material as a hologram. The formation of thistype of holographic grating using the PDLC material described aboveresults in at least the following advantages which are critical toswitching success in the telecommunications as well as other opticallybased industries. H-PDLC gratings offer reduced loss, reducedscattering, reduced crosstalk, polarization independence, operation at1.3 and 1.5 microns, operation at lower voltages, reduction intemperature dependence, and increased reliability and stability,particularly as compared to mechanical devices.

By way of example, the building block for an optical switch isillustrated in FIG. 1. This is a switchable Bragg transmission grating10. An incident beam of light 12 is deflected by a diffraction grating14 over a considerable angle that is equal to twice the Bragg angle forthe wavelength of incident light, producing a diffracted exit beam 16.In this configuration, the grating is usually said to be “on.” With theapplication of a voltage across transparent electrodes(indium-tin-oxide, ITO) (not shown), the index modulation of thehologram vanishes, and the hologram is thus referred to as beingswitched “off.” The exit beam 18 thus propagates through the hologramundeflected. These relatively simple devices could be used to formoptical add/drop boxes or “tunable” laser sources that could be used toreplace a defective laser of any wavelength. The switchable Braggtransmission grating shown in FIG. 1 as well as the optical switchesdescribed throughout this specification may be constructed to operate inan inverse mode wherein the grating is “on” (e.g., the switch is in adiffractive state) with the application of, for example, a voltage andthe grating is “off” (e.g., the switch is not in a diffractive state)when, for example, no voltage is being applied. One skilled in theoptical switching art appreciates the steps necessary to attain thisinverse mode of switching operation.

In FIG. 1, we show incident and diffracted beams with two differentpolarization states: (a) perpendicular to the plane containing theincident, diffracted, and grating wavevectors (commonly known ass-polarization); and (b) in this plane (commonly known asp-polarization). It is well known that for an ordinary grating,s-polarized light will have a stronger coupling (and hence largerdiffraction efficiency) than p-polarized light. The reason for this isthat there is a complete overlap of the electric field vectors for theincident and diffracted waves for s-polarization independent of angle ofincidence. The overlap of p-polarized beams depends on the angle betweenthe two beams, going from complete overlap for 0° angle to zero overlapfor a 90° angle. Hence, for an ordinary grating, the diffractionefficiency of p-polarized light should never exceed that of s-polarizedlight.

In particular, a preferred embodiment of the present inventioncontemplates the formation of a Bragg-type grating using a PDLCmaterial. FIG. 4 is a cross-sectional view of a PDLC Bragg grating 36formed of a layer 50 of the PDLC material, with grating 56 formedtherein, sandwiched between a pair of indium-tin-oxide (ITO) coatedglass slides 52 a and 52 b and between spacers 54.

In the exemplary embodiment wherein the grating hologram 36 is formedfrom PDLC material, the interior of grating hologram 36 reveals a Braggtransmission grating 56 formed when layer 50 was exposed to aninterference pattern from two intersecting beams of coherent laserlight. In FIG. 5, a recording system 60 there is shown as an exemplaryset-up for recording a transmission hologram using PDLC materials of thepresent invention. A coherent light source 62 (e.g., Ar ion laser) isincident upon a spatial filter 64 and a collimating lens 66 prior tobeing divided via a dual slit aperture 68 and impinging upon a prism 70causing the dual beams to interfere within the layer of PDLC material50. Further within this set-up, similar to FIG. 4, the PDLC material issandwiched between layers of ITO glass slides 52 a and 52 b, separatedby spacers 54. Also, in order to insure optical homogeneity, neutraldensity filters 57 were placed before slide 52 a and after slide 52 b,separated by index matching fluid 53. Finally, in order to allow controlof the liquid crystal orientation within the PDLC material, electrodes55 are provided in electrical contact with the ITO glass slides 52 a and52 b. Similarly, one skilled in the art will appreciate the variationsand additions of reflective material necessary to form a Braggreflection grating as opposed to a transmission grating.

The polymer-dispersed liquid crystal material is a mixture of liquidcrystal and prepolymer material homogenized to a viscous solution bysuitable means (e.g., ultrasonification) and spread betweenindium-tin-oxide (“ITO”) coated glass slides with spacers of nominallyabout 10–100 μm thickness and, preferably, about 10–20 μm thickness. TheITO is electrically conductive and serves as an optically transparentelectrode. Preparation, mixing and transfer of the prepolymer materialonto the glass slides are preferably done in the dark as the mixture isextremely sensitive to light. Gratings are typically recorded using the488 nm line of an Argon ion laser with intensities of between about0.1–100 mW/cm² and typical exposure times of 30–120 seconds. The anglebetween the two beams is varied to vary the spacing of the intensitypeaks, and hence the resulting grating spacing of the hologram.Photopolymerization is induced by the optical intensity pattern. A moredetailed discussion of exemplary recording apparatus can be found in R.L. Sutherland, et al., “Switchable holograms in new photopolymer-liquidcrystal composite materials,” Society of Photo-Optical InstrumentationEngineers (SPIE), Proceedings Reprint, Volume 2404, reprinted fromDiffractive and Holographic Optics Technology II (1995), incorporatedherein by reference.

In an embodiment of the present invention, a Bragg grating of FIG. 4constructed with the PDLC material described herein, exhibits theopposite diffraction efficiency characteristics from those recited withreference to FIG. 1, namely, the diffraction efficiency of p-polarizedlight always exceeds that of s-polarized light. Therefore, in the typeof PDLC grating considered in FIG. 1, there is a built-in anisotropythat favors diffraction of light polarized in the plane containing thewavevectors and the grating vector, even though the overlap of fieldvectors is smaller for this case than for the perpendicularpolarization.

In this embodiment of the present invention, the liquid crystal phaseseparates as uniaxial domains 20 with symmetry axis pointedpreferentially along the grating vector 22 as shown in FIG. 6. Theresulting domain 20 has an extraordinary index of refraction n_(e) alongthis symmetry axis, and a smaller ordinary refractive index n_(o)perpendicular to the axis. Since p-polarized light has a component ofits electric field along the symmetry axis, it sees a refractive indexheavily weighted by n_(e), and thus sees a relatively large indexmodulation. On the other hand, s-polarized light sees a refractive indexweighted more by n_(o), and hence experiences a relatively small indexmodulation (n_(e)>n_(o)). The diffraction efficiency of s-polarizedlight is considerably weaker than that of p-polarized light.

The symmetry axes of liquid crystal domains 20 are not perfectly alignedwith the grating vector 22. There is some small statistical distribution25 of the axes about this direction. The average of the statisticaldistribution 25 points along the grating vector 22 as shown in FIG. 7.Thus, s-polarized light will see a small amount of n_(e) mixed in withn_(o), which is what gives it its weak but measurable diffractionefficiency. When a strong electric field 24 is applied perpendicular tothe plane of the grating, as shown in FIG. 8, nearly all liquid crystalsreorient in a direction along the beam propagation for some field value,and both s- and p-polarized light see the same index in the liquidcrystal domains 20, approximately equal to n_(o). Since this indexnearly matches the index of the surrounding polymer, the indexmodulation for both polarization states disappears and the grating issaid to be switched off. When the field strength is further increased,the liquid crystals will eventually orient parallel to the field andthus not be in an orientation to yield zero index modulation. Hence, thediffraction efficiency goes through a minimum near zero and thenincreases slightly with increasing field.

In still more detailed embodiments of the present invention, switchablereflection gratings can be formed using positive Δ∈ liquid crystals. Asshown in FIG. 9 a, such gratings are formed by exposing the PDLCstarting material to a magnetic field during the curing process. Themagnetic field can be generated by the use of Helmholtz coils (as shownin FIG. 9 a), the use of a permanent magnet, or other suitable means.Preferably, the magnetic field M is oriented parallel to the frontsurface of the glass plates (not shown) that are used to form thegrating 140. As a result, the symmetry axis 146 of the liquid crystalswill orient along the field while the mixture is fluid. Whenpolymerization is complete, the field may be removed and the alignmentof the symmetry axis of the liquid crystals will remain unchanged. (SeeFIG. 9 b.) When an electric field is applied, as shown in FIG. 9 c, thepositive Δ∈ liquid crystal will reorient in the direction of the field,which is perpendicular to the front surface of the grating and to theperiodic channels of the grating.

In a second exemplary embodiment, FIG. 10 a depicts a slantedtransmission grating 148 and FIG. 10 b depicts a slanted reflectiongrating 150. A holographic transmission grating is considered slanted ifthe direction of the grating vector K is not parallel to the gratingsurface. In a holographic reflection grating, the grating is said to beslanted if the grating vector K is not perpendicular to the gratingsurface. Slanted gratings have many of the same uses as nonslantedgratings such as visual displays, mirrors, line filters, opticalswitches, and the like.

Primarily, slanted holographic gratings are used to control thedirection of a diffracted beam. For example, in reflection holograms aslanted grating is used to separate the specular reflection of the filmfrom the diffracted beam. In a PDLC holographic grating, a slantedgrating has an even more useful advantage. The slant allows themodulation depth of the grating to be controlled by an electric fieldwhen using either tangential or homeotropic aligned liquid crystals.This is because the slant provides components of the electric field inthe directions both tangent and perpendicular to the grating vector. Inparticular, for the reflection grating, the LC domain symmetry axis willbe oriented along the grating vector K and can be switched to adirection perpendicular to the film plane by a longitudinally appliedfield E. This is the typical geometry for switching of the diffractionefficiency of a slanted reflection grating.

When recording slanted reflection gratings, it is desirable to place thesample between the hypotenuses of two right-angle glass prisms. Neutraldensity filters can then be placed in optical contact with the backfaces of the prisms using index matching fluids so as to frustrate backreflections which would cause spurious gratings to also be recorded. Theincident laser beam is split by a conventional beam splitter into twobeams which are then directed to the front faces of the prisms, and thenoverlapped in the sample at the desired angle. The beams thus enter thesample from opposite sides. This prism coupling technique permits thelight to enter the sample at greater angles. The slant of the resultinggrating is determined by the angle at which the prism assembly isrotated (i.e., the angle between the direction of one incident beam andthe normal to the prism front face at which that beam enters the prism).

The exposure times and intensities can be varied depending on thediffraction efficiency and liquid crystal domain size desired. Liquidcrystal domain size can be controlled by varying the concentrations ofphotoinitiator, coinitiator and chain-extending (or cross-linking)agent. The orientation of the nematic directors can be controlled whilethe gratings are being recorded by application of an external electricfield across the ITO electrodes.

The scanning electron micrograph shown in FIG. 2A of the referencedApplied Physics Letters article and incorporated herein by reference isof the surface of a grating which was recorded in a sample with a 36 wt% loading of liquid crystal using the 488 nm line of an Argon ion laserat an intensity of 95 mW/cm². The size of the liquid crystal domains isabout 0.2 μm and the grating spacing is about 0.54 μm. This sample,which is approximately 20 μm thick, diffracts light in the Bragg regime.

FIG. 11 is a graph of the normalized net transmittance and normalizednet diffraction efficiency of a hologram made according to the teachingsof the present invention versus the root mean square voltage (“Vrms”)applied across the hologram. Δη is the change in first-order Braggdiffraction efficiency. ΔT is the change in zero-order transmittance.FIG. 11 shows that energy is transferred from the first-order beam tothe zero-order beam as the voltage is increased. There is a true minimumof the diffraction efficiency at approximately 225 Vrms. The peakdiffraction efficiency can approach 100%, depending on the wavelengthand polarization of the probe beam, by appropriate adjustment of thesample thickness. The minimum diffraction efficiency can be made toapproach 0% by slight adjustment of the parameters of the PDLC materialto force the refractive index of the cured polymer to equal the ordinaryrefractive index of the liquid crystal.

By increasing the frequency of the applied voltage, the switchingvoltage for minimum diffraction efficiency can be decreasedsignificantly. This is illustrated in FIG. 12, which is a graph of boththe threshold rms voltage 21 and the complete switching rms voltage 23needed for switching a hologram made according to the teachings of thepresent invention to minimum diffraction efficiency versus the frequencyof the rms voltage. The threshold and complete switching rms voltagesare reduced to 20 Vrms and 60 Vrms, respectively, at 10 kHz. Lowervalues are expected at even higher frequencies.

Smaller liquid crystal droplet sizes have the problem that it takes highswitching voltages to switch their orientation. As described in theprevious paragraph, using alternating current switching voltages at highfrequencies helps reduce the needed switching voltage. As demonstratedin FIG. 13, another unique discovery of the present invention is thatadding a surfactant (e.g., octanoic acid) to the prepolymer material inamounts of about 4%–6% by weight of the total mixture resulted in sampleholograms with switching voltages near 50 Vrms at lower frequencies of1–2 kHz. As shown in FIG. 14, it has also been found that the use of thesurfactant with the associated reduction in droplet size, reduces theswitching time of the PDLC materials. Thus, samples made with surfactantcan be switched on the order of 25–44 microseconds.

Thermal control of diffraction efficiency is illustrated in FIG. 15, agraph of the normalized net transmittance and normalized net diffractionefficiency of a hologram made according to the teachings of the presentinvention versus temperature.

The polymer-dispersed liquid crystal materials described hereinsuccessfully demonstrate the utility for recording volume holograms of aparticular composition for such polymer-dispersed liquid crystalsystems. Although the disclosed polymer-dispersed liquid crystal systemsare specialized, embodiments of the present invention will findapplication in other areas where a fast curing polymer and a materialthat can be phase-separated from the polymer will find use.

A switchable hologram is one for which the diffraction efficiency of thehologram may be modulated by application of an electric field, and canbe switched from a fully on state (high diffraction efficiency) to afully off state (low or zero diffraction efficiency). A static hologramis one whose properties remain fixed independent of an applied field. Inanother embodiment of the present invention, PDLC materials can be madethat exhibit a property known as form birefringence whereby polarizedlight that is transmitted through the grating will have its polarizationmodified. Such gratings are known as subwavelength gratings, and theybehave like a negative uniaxial crystal, such as calcite, potassiumdihydrogen phosphate, or lithium niobate, with an optic axisperpendicular to the PDLC planes. Referring now to FIG. 16, there isshown an elevational view of a transmission grating 200 in accordancewith the present invention having periodic planes of polymer channels200 a and PDLC channels 200 b disposed perpendicular to the frontsurface 204 of the grating 200. The optic axis 206 is disposedperpendicular to polymer planes 200 a and the PDLC planes 200 b. Eachpolymer plane 200 a has a thickness t_(p) and refractive index n_(p),and each PDLC plane 200 b has a thickness t_(PDLC) and refractive indexn_(PDLC).

Where the combined thickness of the PDLC plane and the polymer plane issubstantially less than an optical wavelength (i.e.,(t_(PDLC)+t_(p))<<λ), the grating will exhibit form birefringence. Asdiscussed below, the magnitude of the shift in polarization isproportional to the length of the grating. Thus, by carefully selectingthe length, L, of the subwavelength grating for a given wavelength oflight, one can rotate the plane of polarization or create circularlypolarized light. Consequently, such subwavelength gratings can bedesigned to act as a half-wave or quarter-wave plate, respectively.Thus, an advantage of this process is that the birefringence of thematerial may be controlled by simple design parameters and optimized toa particular wavelength, rather than relying on the given birefringenceof any material at that wavelength.

To form a half-wave plate, the retardance of the subwavelength gratingmust be equal to one-half of a wavelength, i.e., retardance=λ/2, and toform a quarter-wave plate, the retardance must be equal to one-quarterof a wavelength, i.e. retardance=λ/4. It is known that the retardance isrelated to net birefringence, |Δn|, which is the difference between theordinary index of refraction, n_(o), and the extraordinary index ofrefraction n_(e), of the sub-wavelength grating by the followingrelation:Retardance=|Δn|L=|n _(e) −n _(o) |L

Thus, for a half-wave plate, i.e., a retardance equal to one-half of awavelength, the length of the subwavelength grating should be selectedso that:L=λ/(2|Δn|)

Similarly, for a quarter-wave plate, i.e., a retardance equal toone-quarter of a wavelength, the length of the subwavelength gratingshould be selected so that:L=λ/(4|Δn|)

If, for example, the polarization of the incident light is at an angleof 45° with respect to the optical axis 210 of a half-wave plate 212, asshown in FIG. 17 a, the plane polarization will be preserved, but thepolarization of the wave exiting the plate will be shifted by 90°. Thus,referring now to FIGS. 17 b and 17 c, where the half-wave plate 212 isplaced between cross polarizers 214 and 216, the incident light will betransmitted. If an appropriate switching voltage is applied, as shown inFIG. 17 d, the polarization of the light is not rotated and the lightwill be blocked by the second polarizer.

For a quarter wave plate plane polarized light is converted tocircularly polarized light. Thus, referring now to FIG. 18 a, wherequarter wave plate 217 is placed between a polarizing beam splitter 218and a mirror 219, the reflected light will be reflected by the beamsplitter 218. If an appropriate switching voltage is applied, as shownin FIG. 18 b, the reflected light will pass through the beam splitterand be retroreflected on the incident beam.

Referring now to FIG. 19 a, there is shown an elevational view of asubwavelength grating 230 recorded in accordance with theabove-described methods and having periodic planes of polymer channels230 a and PDLC channels 230 b disposed perpendicular to the frontsurface 234 of grating 230. As shown in FIG. 19 a, the symmetry axis 232of the liquid crystal domains is disposed in a direction parallel to thefront surface 234 of the grating and perpendicular to the periodicchannels 230 a and 230 b of the grating 230. Thus, when an electricfield E is applied across the grating, as shown in FIG. 18 b, thesymmetry axis 232 distorts and reorients in a direction along the fieldE, which is perpendicular to the front surface 234 of the grating andparallel to the periodic channels 230 a and 230 b of the grating 230. Asa result, subwavelength grating 230 can be switched between a statewhere it changes the polarization of the incident radiation and a statein which it does not. The direction of the liquid crystal domainsymmetry 232 is due to a surface tension gradient which occurs as aresult of the anisotropic diffusion of monomer and liquid crystal duringrecording of the grating and that this gradient causes the liquidcrystal domain symmetry to orient in a direction perpendicular to theperiodic planes.

As discussed in Born and Wolf, Principles of Optics, 5th Ed., New York(1975) and incorporated herein by reference, the birefringence of asubwavelength grating is given by the following relation:

where

${n_{e}^{2} - n_{o}^{2}} = \frac{- \lbrack {( f_{PDLC} )( f_{p} )( {n_{PDLC}^{2} - n_{p}^{2}} )} \rbrack}{\lbrack {{f_{PDLC}n_{PDLC}^{2}} + {f_{p}n_{p}^{2}}} \rbrack}$n_(o)=the ordinary index of refraction of the subwavelength grating;n_(e)=the extraordinary index of refraction;n_(PDLC)=the refractive index of the PDLC plane;n_(p)=the refractive index of the polymer plane;n_(LC)=the effective refractive index of the liquid crystal seen by anincident optical wave;f_(PDLC)=t_(PDLC)/(t_(PDLC)+t_(p)) andf_(p)=t_(p)/(t_(PDLC)+t_(p))Thus, the net birefringence of the subwavelength grating will be zero ifn_(PDLC)=n_(p).

It is known that the effective refractive index of the liquid crystal,n_(LC), is a function of the applied electric field, having a maximumwhen the field is zero and a value equal to that of the polymer, n_(p),at some value of the electric field, E_(MAX). Thus, by application of anelectric field, the refractive index of the liquid crystal, n_(LC), and,hence, the refractive index of the PDLC plane can be altered. Using therelationship set forth above, the net birefringence of a subwavelengthgrating will be a minimum when n_(PDLC) is equal to n_(p), i.e., whenn_(LC)=n_(p). Therefore, if the refractive index of the PDLC plane canbe matched to the refractive index of the polymer plane, i.e.,n_(PDLC)=n_(p), by the application of an electric field, thebirefringence of the subwavelength grating can be switched off.

The following equation for net birefringence, i.e. |Δn|=|n_(e)−n_(o)|,follows from the equation given in Born and Wolf (reproduced above):

${\Delta\; n} = \frac{ {{- \lbrack f_{PDLC} )}( f_{p} )( {n_{PDLC}^{2} - n_{p}^{2}} )} \rbrack}{\lbrack {2{n_{AVG}( {{f_{PDLC}n_{PDLC}^{2}} + {f_{p}n_{p}^{2}}} )}} \rbrack}$where n_(AVG)=(n_(e)+n_(o))/2

Furthermore, it is known that the refractive index of the PDLC planen_(PDLC) is related to the effective refractive index of the liquidcrystal seen by an incident optical wave, n_(LC), and the refractiveindex of the surrounding polymer plane, n_(p), by the followingrelation:n _(PDLC) =n _(p) +f _(LC) [n _(LC) −n _(p)]where f_(LC) is the volume fraction of liquid crystal dispersed in thepolymer within the PDLC plane, f_(LC)=[V_(LC)/(V_(LC)+V_(P))]V_(LC)=the volume of the liquid crystal andV_(P)=the volume of the polymer.

By way of example, a typical value for the effective refractive indexfor the liquid crystal in the absence of an electric field isn_(LC)=1.7, and for the polymer layer n_(p)=1.5. For a grating where thethickness of the PDLC planes and the polymer planes are equal (i.e.,t_(PDLC)=t_(p), f_(PDLC)=0.5=f_(p)) and f_(LC)=0.35, the netbirefringence, Δn, of the subwavelength grating is approximately 0.008.Thus, where the incident light has a wavelength of 0.8 μm, the length ofthe subwavelength grating should be 50 μm for a half-wave plate and 25μm for a quarter-wave plate. Furthermore, by application of an electricfield of approximately 5 V/μm, the refractive index of the liquidcrystal can be matched to the refractive index of the polymer and thebirefringence of the subwavelength grating turned off. Thus, theswitching voltage, V_(π), for a half-wave plate is on the order of 250volts, and for a quarter-wave plate approximately 125 volts.

By applying such voltages, the plates can be switched between the on andoff (zero retardance) states on the order of microseconds. As a means ofcomparison, current Pockels cell technology can be switched innanoseconds with voltages of approximately 1000–2000 volts, and bulknematic liquid crystals can be switched on the order of millisecondswith voltages of approximately 5 volts.

In an alternative embodiment of the invention shown in FIG. 20, theswitching voltage of the subwavelength grating can be reduced bystacking several subwavelength gratings 220 a–e together, and connectingthem electrically in parallel. By way of example, it has been found thata stack of five gratings each with a length of 10 μm yields thethickness required for a half-wave plate. It should be noted that thelength of the sample is somewhat greater than 50 μm, because eachgrating includes an indium-tin-oxide coating which acts as a transparentelectrode. The switching voltage for such a stack of plates, however, isonly 50 volts.

Subwavelength gratings in accordance with embodiments of the presentinvention find suitable application in the areas of polarization opticsand optical switches for displays and laser optics, as well as tunablefilters for telecommunications, colorimetry, spectroscopy, laserprotection, and the like.

Similarly, in accordance with embodiments of the present invention, ahigh birefringence static sub-wavelength wave-plate can also be formed.Due to the fact that the refractive index for air is significantly lowerthan for most liquid crystals, the corresponding thickness of thehalf-wave plate would be reduced accordingly. Synthesized wave-plates inaccordance with embodiments of the present invention can be used in manyapplications employing polarization optics, particularly where amaterial of the appropriate birefringence at the appropriate wavelengthis unavailable, too costly, or too bulky.

In a first preferred embodiment of the present invention, the PDLCholographic grating elements are components in an optical add/dropmultiplexer (OADM). When inserted into an optical path (e.g., fiber)carrying multiple wavelength division multiplexed (WDM) wavelengths, anOADM performs the function of “dropping” one or more of the wavelengthstreams from the fiber and/or “adding” one or more wavelength streams tothe fiber. It is relatively simple to construct an OADM that drops andadds fixed wavelengths all the time. However, optical networks need theflexibility to respond dynamically to a changing demand profile.Therefore, a remotely reconfigurable OADM is desirable. A schematic fora reconfigurable OADM using the PDLC material is shown in FIG. 21.

Referring to FIG. 21, a preferred embodiment of an OADM system 30 isshown which includes a PDLC switchable fiber Bragg filter 35 formed froma stack of switchable Bragg gratings 36, each of which is set todiffract one of the unique WDM wavelengths inputted thereto via aconventional input device such as a fiber or waveguide. In FIG. 21 arecirculator 37 and a combiner 38 are located on either side of theswitchable Bragg grating filter 35 to complete the OADM system 30. Thedrop function for a particular wavelength is achieved by setting all thefilters to the transparent mode except for the one with thecorresponding wavelength, which is set to opaque. The chosen wavelengthis diffracted while the remaining wavelengths are transmitted throughthe filter without optical interaction. Any wavelength can be added, ofcourse, depending on the wavelength of the laser source for the addedsignal stream.

In an alternative preferred embodiment of the present invention,switchable PDLC gratings are combined to create a variable frequencylaser source for Dense Wavelength Division Multiplexing (DWDM). As thenumber of wavelengths in DWDM systems increases, the provisioning oflaser sources becomes more and more of a problem for carriers. Ideally,one would like to have a tunable laser source so that one laser couldserve as a replacement for a large number of different wavelengthsources. The tunable laser would be tuned to the correct frequency whenit was put into service. The PDLC variable frequency laser source isdiscretely tunable and is very cheap relative to the tunable sourcescurrently under development.

In an embodiment of a DWDM scheme, optical signals at differentwavelengths centered about 1550 nm and separated by 100–200 GHz(˜0.8–1.6 nm) are multiplexed down a single fiber or waveguide. A filteris desired that can select one wavelength out of this set. A Braggfilter is ideal for this, because it will selectively reflect light at aspecific wavelength. However, another requirement is that it have anarrow bandwidth so that it does not partially reflect another nearbywavelength, producing loss and crosstalk between channels. The bandwidthof a Bragg filter can be estimated by Δβ˜κ, where Δβ=β−|K|/2,β=2πn_(eff)/λ,n_(eff) is the effective mode refractive index, K is thegrating vector, and κ is the grating coupling constant. The latter canbe given by κ=πn_(l)/λ, where n₁ is the grating index modulation. Thebandwidth relation can be recast in terms of frequency bandwidth Δν(Δν/ν=Δβ/β), which sets the size of n₁ by the wavelength separationrequirement of the DWDM scheme. Thus, n₁˜2n_(eff)λΔν/c, where c is thespeed of light. For n_(eff)≈1.5, λ≈1550 nm, and Δν=100–200 GHz, we havethe requirement that n₁˜0.0015–0.0030. This is about one order ofmagnitude smaller than the index typically achieved in switchable PDLCholograms (n₁˜0.02–0.05). The reflection efficiency can be estimatedfrom R≈tanh² (κL), where L is the physical length of the filter. Giventhe above requirement on index modulation, for R=0.9999, we must haveL˜0.9–1.8 mm. For R=0.99999, L˜1.1–2.2 mm. This is very thick comparedto typical PDLC holograms that are typically ˜10 μm thick. Therefore,two requirements of the switchable Bragg filter in these DWDMapplications are that it (1) be physically thick, and (2) have smallindex modulation.

A concept for a switchable Bragg filter in a channel waveguide thatmeets the aforementioned requirements is described as shown in FIGS. 22a–d. In a process for forming a channel waveguide Bragg filter an emptychannel of waveguide dimensions known to those skilled in the art, isetched in a glass or polymer substrate of the appropriate refractiveindex (FIG. 22 a). Next, the channel is filled with a pre-polymer/liquidcrystal (“PPLC”) material (FIG. 22 b). As described above, this PPLCmaterial, once exposed, becomes the PDLC material. The PPLC material isexposed to two coherent laser beams from the same side of the substrateas shown in FIG. 22 c. Finally, upon curing, a holographic PDLC gratingis formed with grating vector K parallel to the channel axis asillustrated in FIG. 22 d. In a preferred embodiment of the presentinvention, the grating period is Λ˜0.5 μm, and the total length of thefilter is L˜1–2 mm. For light propagated down this waveguide, thisgrating looks like a reflection grating, and light of wavelengthλ=2n_(eff)Λ˜1500 nm will match the Bragg condition and be selectivelyreflected, for n_(eff)˜1.5.

Light propagating down the channel waveguide will see liquid crystaldomains 20 with symmetry axes pointing primarily along the gratingvector 22, but with a small statistical distribution 25 about thisdirection as shown in FIG. 23. With the azimuthally symmetricdistribution of symmetry axes shown in FIG. 23, both TE and TM polarizedlight will see the same admixture of indices n_(o), and n_(e) in theliquid crystal domains 20, on average. Hence, the index modulation seenby light is polarization insensitive, and no polarization diversityscheme is needed for this device. Moreover, since the liquid crystaldomain index seen by light is weighted most heavily by n_(o) and onlypartially by n_(e), the resulting index modulation will be small. Thus,the two requirements of the Bragg filter are met: a small indexmodulation and a physically thick filter. In addition, the reflectionefficiency of this filter will be polarization insensitive.

In a preferred embodiment of the present invention, the switching of thePDLC Bragg filters is accomplished by supplying and removing a voltageto the filter via a voltage source. Finger electrodes 27 attached to thefilter are deposited in the formation illustrated in FIG. 24. These canbe made using standard gold, aluminum, ITO, or other electrode materialsknown to those skilled in the art. Similarly, any known depositionmethod may be used to deposit the electrode material, e.g., sputteringor lithography. In this embodiment, the fingers are deposited adjacentto the waveguide 26 and adjacent to the pure polymer regions 28 of thePDLC grating as opposed to the PDLC regions 29 of the waveguide 26.Opposite polarity voltages are applied to every other electrode 27 onthe same side of the waveguide 26, and same polarity voltages areapplied to electrodes 27 directly opposite one another on opposite sidesof the waveguide 26. If alternating current (“ac”) voltage waveforms areused, then every other electrode is grounded, and all other electrodesare “hot” (same or opposite polarity). Electrodes 27 on the oppositeside of the waveguide 26 are addressed electrically in an identicalmanner. This produces the electric field lines as shown. The fringefield pattern of electrodes 27 on opposite sides of the waveguide 26super-pose to form a longitudinal field in the PDLC regions 29 of thefilter. This causes the droplet axes to line up in the direction of thegrating vector, and the index modulation seen by light switches to zeroas shown in FIG. 25 a–b. Thus, the filter is switched off.

In a further embodiment of the present invention, multiple individualBragg filters are concatenated as shown in FIG. 26. Each filter has aslightly different period Λ, set to cause reflection of light centeredat one of the DWDM wavelengths. With all of the filters powered exceptthe one with period Λj, all wavelengths pass except λj, which isretroreflected back along the waveguide. Alternatively, with all filtersunpowered except the one with period Λj, all wavelengths areretroreflected except λj, which is passed along the waveguide.

The finger electrode pattern can be fabricated simultaneously with theholographic filter recording in a manner outlined in FIGS. 27 a–b. Inthis process, the selected electrode material is coated uniformly ontothe non-etched regions of the glass or polymer substrate. Next, theelectrode materials are coated uniformly with a negative photoresist.The negative photoresist is exposed to the interfering incident beamsand the photoresist is consequently exposed to alternating light anddark regions. As a result of this exposure, alternating regions will besusceptible to etching processes so as to form the finger electrodes.Prior to laser exposure, the etched channel is filled with aprepolymer/liquid crystal material as shown in FIG. 22 b. The entirestructure is then exposed holographically using two coherent laser beamsfrom the same side of the substrate, as illustrated in FIG. 27 a. Withsubsequent lithographic processing, the areas of the electrodes exposedto bright fringes remain while the material in the dark fringes isremoved, forming the desired finger electrode structure shown in FIG. 27b, with electrodes adjacent to pure polymer regions in the filter.Alternatively, a positive photoresist may be used, such as ShipleyMegaposit® SPR®3000 series. These may be exposed in the blue and canproduce line features <0.4 μm. In this case, a phase mask is placed infront of one of the recording beams. For example, this could be twopieces of suitably thick glass placed over the part of a beam incidenton the electrode materials. The purpose of the phase mask is to shiftthe phase of light in the electrode regions so that the fringes shiftspatially by 180° with respect to the channel region. After processing,this would yield the same pattern as shown in FIG. 27 b. Precaution mayneed to be taken during this process to ensure the PDLC filter/waveguideregion is not damaged. This is one switching technique. However, anyscheme that will yield a longitudinal field along the grating vectorwill work for this device.

Long-period fiber gratings, with Λ>>λ, have been employed as wavelengthselective attenuators in DWDM applications. These can be used, forexample, to flatten the gain spectrum of an Er-doped fiber laser [see,e.g., A. M. Vengsarkar et al., Opt. Lett. 21, 336 (1996)]. These are aseries of different static gratings that must be pre-recorded toprecisely match the inverted gain profile of the Er laser whenconcatenated. Long-period fiber gratings produce wavelength selectiveloss by coupling radiation at a specific wavelength from a guided modeto a cladding mode. The cladding modes are very lossy. Thus, the opticalsignal at the selected wavelength is attenuated.

In an alternative embodiment of the present invention, a wavelengthselective attenuator whose properties can be finely adjusted by anexternal stimulus (e.g., an electric field) to control the attenuationis shown. The variable attenuator is formed using a PDLC switchablehologram. A channel waveguide PDLC grating is fabricated as describedabove with reference to FIGS. 22( a)–(d) and FIGS. 27( a)–(b). However,the two laser beams are directed in a symmetrical way at relativelysmall angles with respect to the normal of the glass or polymersubstrate as illustrated in FIG. 28( a). In an embodiment utilizing thislaser exposure configuration, the resulting grating period is Λ=λ/2n sinθ, where λ is the recording wavelength. For λ=532 nm, n˜1.5, an angle ofθ˜0.1° will make a grating with period Λ˜100 μm. Alternatively, anamplitude mask may be used, as shown in FIG. 28( b), with the maskflood-loaded with a single beam of the recording light. In theembodiment using an amplitude mask, it may be desirable to cover thePDLC channel waveguide with another substrate prior to recording to forma symmetrical structure. Depositing finger electrodes as described withreference to FIGS. 27( a)–(b) above completes the device.

The phase-matching condition for coupling light from the core (channel)to the cladding (substrate) mode is given by n_(co)−n_(cl)=λ/Λ, where“co” stands for core, “cl” stands for cladding, n_(co), and n_(cl) arethe effective core and cladding refractive indices, respectively, at,for example, wavelength ˜1550 nm, and Λ˜100 μm. The strength of thecoupling, or equivalently, the transmission loss through the grating, isdetermined by the index modulation of the grating. Through theapplication of electric fields as described above, the index modulationcan be varied in a continuous manner from maximum at zero field to zeromodulation at a field where droplet symmetry axes are all aligned alongthe grating vector.

In an embodiment of the present invention, several long-period PDLCgratings centered at wavelengths λ_(i) are concatenated as shown in FIG.29. For example, these could be wavelengths in the gain spectrum of anEr laser. However, the use of the device is not limited to thisapplication. By applying different voltages to each grating, the desiredspectral shape of the transmission loss filter can be achieved.

An alternative method of fabricating a voltage-controlled long-periodPDLC grating is illustrated in FIGS. 30( a)–(b). A PDLC grating isfabricated using the process described with reference to FIG. 27(a)–(b), but with a short period that is well outside the Bragg regimefor, by way of example, λ˜1550 nm (e.g., Λ≦0.4 μm or Λ≧0.6 μm, withn_(eff),˜1.5). The grating is made several millimeters in length andfinger electrodes may be applied as described with reference to FIG. 27(a)–(b). Light will not be diffracted by this index modulation, but willsee an effective index that will change as a voltage is applied to theelectrodes. A different voltage is applied to each finger electrode insuch a manner that the voltage profile is periodic along the channelwaveguide with period ˜100 μm. This will electro-optically induce along-period grating in the waveguide as shown. In an alternativeembodiment, a complex transmission spectrum is produced where thedesired spectral shape is Fourier analyzed, and the resultingcombination of periodic voltages superposed on the electrodes, yieldingthe desired transmission spectrum.

In an alternate embodiment shown in FIG. 31( a)–(b), a method and systemfor fabricating a switchable Bragg grating (either short-period orlong-period) coupled to two optical fibers is illustrated. Referring toFIG. 31( a), a pre-polymer/liquid crystal holographic material 50 isformed in a cylindrical geometry, such that when the material 50 ispolymerized, a switchable Bragg grating is formed that is in line withtwo optical fibers 110 and 112, respectively. In this embodiment, thetwo fibers are fitted with graded refractive index (GRIN) lenses 114 and116. A first GRIN lens 114 out-couples and collimates light from a firstfiber 110 into the switchable Bragg grating formed from material 50. Asecond GRIN lens 116 collects and couples light into a second fiber 112.These GRIN lenses are optically contacted to the ends of the fibersusing an optical adhesive. The two optical fiber/GRIN lenses 114 and 116are inserted into a hollow capillary tube 118, leaving a gap 120 for theBragg grating, on the order of a few millimeters in length. Thepre-polymer/liquid crystal holographic material 50 is injected, e.g., bya syringe 122, into the gap 120 so that it fills the gap and makesoptical contact with the GRIN lenses 114 and 116. The geometry of thecapillary tube 118 is not limited, and may be, for example, cylindricalor rectangular. In a specific embodiment, the capillary could be fittedwith a fill port over the region of the gap between the two fibers tofacilitate the injection of the pre-polymer/liquid crystal material 50.

The previously prepared pre-polymer/liquid crystal material 50 is thensandwiched between two optical flats 124 (e.g., glass plates), as shownin FIG. 31( b). The flats 124 are clamped together, and an indexmatching solution 126 is injected between the flats which fills all gapsin the structure. One purpose of this is to present an optically flatmedium to incident light, with no index-mismatched surfaces that wouldproduce spuriously scattered light. This structure is then irradiatedwith two coherent beams of light 128 incident on the same side of thestructure. The pre-polymer/liquid crystal material 50 absorbs the light,and the ensuing polymerization produces a holographic PDLC Bragg gratingwith grating vector along the fiber axes. The average index ofrefraction of the Bragg grating thus formed will be substantially equalto the index of refraction of the fiber core and GRIN lens materials. Ina further specific embodiment, the inner surface of the capillary inFIG. 31( a)–(b) is treated with a release agent so that the capillarycan be removed (e.g., split and detached) after the Bragg gratingfabrication is completed.

Referring to FIG(s), 31(c)–(d), a system and method for switching theholographic PDLC Bragg grating of FIG. 31( a)–(b) is nearly identical tothe system described with reference to FIG. 24. As with the channelgeometry, it is desirable to orient the liquid crystal symmetry axes ina direction along the grating vector (which is also the direction oflight propagation) to switch the grating off. The fiber/grating assembly130 is positioned in a groove etched in a glass or polymer block 132 andclamped in place (clamps not shown). Finger electrodes 134 are depositedon the surface of the block 132 as shown. Finger electrodes 134 areaddressed in the manner described with reference to FIG. 24, such that alongitudinal electric field is established in the Bragg grating,parallel to the propagation axis/grating vector and the Bragg grating isswitched off.

In an alternate embodiment, the polymerized PDLC grating is etchedand/or polished to a dimension that is commensurate with the core of thefiber (˜10 μm for single-mode fibers) and the finger electrodes areplaced in close proximity to the reduced structure. This configurationmay lower the applicable voltage requirement for establishing thecritical field for switching the liquid crystal droplets. Techniques arewell known in the art for polishing glass fibers in this way to exposethe core. Another advantage to reducing the dimension of the PDLCgrating is the resulting total internal reflection (TIR) which occurs atthe grating-air interface. The occurrence of TIR increases thepropagation and coupling efficiency of the incoming beam into thereceiving fiber.

An alternative switching method and system, compatible with thecylindrical geometry of fibers, is illustrated in FIG. 32. An electriccurrent-carrying coil of wire 150 is wrapped tightly (more tightly thanillustrated) around the cylindrical switchable Bragg grating 152,forming a magnetic solenoid. Current is applied to the solenoid throughpower supplies, establishing a magnetic field along the axis of the coil150 that is collinear with the axes of the fibers and the Bragg grating152. Due to the magnetic anisotropy of the liquid crystal(Δχ=χ_(∥)−χ_(⊥), where χ_(∥) and χ_(⊥) are the magnetic susceptibilitiesparallel and perpendicular to the liquid crystal droplet symmetry axis,respectively), the droplet axes align preferentially with the appliedmagnetic field H. This produces the same effect that a longitudinalelectric field would produce, and switches the filter off in a manneranalogous to that illustrated in FIG. 25( b), where the E field isreplaced by the H field. In this switching method, field strength isindependent of the coil diameter, and is consequently independent of thegrating diameter. The field strength depends on the current in the coiland the number of turns of the coil per unit length.

A further alternative switching method and system for a cylindricalgrating is illustrated in FIG. 33. The switchable grating 160 ispositioned between at least two heater blocks (e.g., top 162 and bottom164), and as many as four heater blocks (e.g., top and bottom and bothsides) (not shown). The electrically driven heater blocks 162 and 164(e.g., ceramic resistors) heat the grating 160 when power is applied tothem. This raises the temperature of the liquid crystal droplets in thegrating 160, lowering their effective refractive index as temperatureincreases. Near or at the critical temperature for transition from thenematic phase to the isotropic phase, the index of refraction of theliquid crystal substantially matches that of surrounding polymer, thuscausing the index of refraction modulation to vanish and the grating toswitch off. When power is removed from the heaters, the liquid crystalscool back to their nematic state, and the original index of refractionmodulation is restored; the grating is switched back on. Due to thesmall thermal mass of the grating, this switching is quite rapid.

Referring to FIG. 34( a), a discretely tunable laser 40 is shown whichcomprises a multiple wavelength, multi-mode laser source 42 and aswitchable Bragg grating filter 35 formed from a stack of switchableBragg gratings 36. In this preferred embodiment, the multiplewavelength, multi-mode laser source 42 could be, for example, aFabry-Perot semiconductor laser. Fabry-Perot semiconductor lasers areless expensive relative to single mode lasers and they emit light atseveral discrete wavelengths that can be adjusted so that these discretewavelengths correspond to the International Telecommunications Union(ITU) grid of wavelengths for DWDM. On the order of 10 differentwavelengths can be generated from such a source, so it would requireseveral sources to cover the entire DWDM spectrum.

The light emitted from the multiple wavelength, multi-mode laser 42 isdirected into a switchable fiber Bragg filter 35 similar to the onedescribed earlier with reference to the OADM system 30. In this case,the desired wavelength can be passed through while all the others arediffracted elsewhere. Alternatively, as shown in FIG. 34( b) eachwavelength could be diffracted into a specific direction and picked upby a respective output device 44 (e.g., detector, fiber or waveguide).The output device 44 is alternatively configured to receive only asingle wavelength in a single mode, a single wavelength in multiplemodes, multiple wavelengths all in the same mode, or multiplewavelengths in multiple modes.

In further preferred embodiments of the present invention, a N²×N²optical cross-connect switch consisting of 2N principal layers of H-PDLCmatrix holographic switches is presented. One purpose of a cross-connectswitch is to direct an optical signal from any element of a rectangularinput matrix to any element of an identical output matrix. This can beaccomplished by a series of up-down and right-left moves. All inputs andoutputs are assumed to be parallel.

The beam deflector system shown in FIG. 35 can achieve a simple one-stepmove. This device is referred to as an “up” deflector because theincident beam 32 is deflected up, forming an intermediate beam 33, by afirst PDLC holographic grating 36. A second identical “up” holographicgrating 36 positioned some distance away accepts this beam and producesan exit beam 34, parallel to the incident beam 32. Thus, these twolayers, a deflector and acceptor, form a one-step move, i.e., taking aparallel input, moving it up one space, and producing a parallel output.

The same holographic grating can be used for the “down,” “right,” and“left” deflections. If the holographic grating 36 in FIG. 35 is rotatedby 180° about its normal axis, it becomes a “down” deflector. Likewise,if it is rotated by ±90°, it becomes a “right” or “left” deflector.Hence, a single type of hologram achieves all of the necessarydeflections.

The switches in this concept are transmission holographic switches thateither deflect up/down or right/left. Thus, based on the discussionabove, the incident light should be polarized in the vertical plane orthe horizontal plane, respectively, for optimum efficiency (and henceminimum loss and crosstalk). The scheme shown in FIG. 36 achieves thisas well as providing for polarization diversity when the input light isunpolarized (as is often the case for light emitted from diode lasers oroutcoupled from an optical fiber). This configuration has two identicaloptical paths including a polarization beam Splitter (PBS), up/down(U/D) and right/left (R/L) deflector/acceptor layers, mirror, and ahalf-wave (λ/2) plate, with the exception that the order of the up/downand right/left layers is reversed in each path. At the first PBS, thelight is split into two beams of orthogonal polarization, which aredirected at 90° to one another. The beam in the lower path is verticallypolarized and hence achieves optimum diffractive coupling in the U/Dlayers. The intermediate switched outputs from the U/D layers aredirected through the λ/2 plate, which rotates the polarization tohorizontal. This is the polarization for optimum diffractive coupling inthe R/L layers. The outputs of the R/L layers, still having horizontalpolarization, are directed by the mirror to the final PBS where they arereflected out of the system. Light incident on the upper path ishorizontally polarized and as such it is optimally polarized for the R/Llayers. The λ/2 plate rotates the polarization of the intermediateoutput in the upper path to vertical, which is optimally polarized forthe subsequent upper path U/D layers. The net switching is identical tothe lower path since the R/L and U/D operations are done independently.The final outputs from the upper path pass directly through the finalPBS and recombine with the signals from the lower path in a preciselysynchronized fashion since the two legs are completely symmetric. Thus,this polarization diversity scheme utilizes the polarization propertiesof PDLC transmission gratings to achieve a matrix cross-connect switchwith optimum efficiency and hence minimum crosstalk and polarizationdependent loss.

The N²×N² matrix cross-connect switch is unique. Most cross-connectswitches discussed in the literature (see, e.g., Ramaswami andSivarajan, 1998) are linear N×N switches. Actually, any set of columnsin the U/D layers or any set of rows in the R/L layers constitute an N×Nswitch and can be discussed as such. These N×N cross-connect switchesare wide-sense nonblocking in that, for the given paths defined, anyunused input can be connected to any unused output without requiring anyexisting connections to be broken. This can be seen in that (a) thedirections of signals to their respective outputs are determined bydefections set in the very first layer (none of these conflict with oneanother), and (b) the output of a signal in any channel is selected byactivating a hologram that is Bragg-matched only to the desireddirection selected in the first layer. Any other signal passing throughthat hologram is not Bragg-matched and hence not deflected. This type ofN×N cross-connect switch, which we will designate by SU, requires2[(N−1)²+1] elementary switches.

Another type of wide-sense nonblocking N×N switch that has beendiscussed in the literature is a crossbar (CB) switch. This type ofswitch requires only N² elementary switches. A 4×4 crossbar switch madewith down deflector, switchable PDLC transmission holograms is shown inFIG. 37. This can easily be generalized to an N×N switch for any N. Inthe powered state, the elementary switches directly transmit light,while in the unpowered state they deflect or accept a beam of light. Thedashed paths in FIG. 37 show the potential connections of inputs tooutputs. The heavy line shows an actual connection from input 1 tooutput 3. We note that this type of linear cross-connect can begeneralized to form parallel columns of down or up deflectors which arethen coupled to a set of parallel rows of right or left deflectors in aplane perpendicular to the plane of FIG. 37 to form an N²×N² matrixcross-connect switch. The same polarization diversity scheme illustratedin FIG. 36 could then be employed.

For these wide-sense nonblocking switches, a path or paths may be foundthat make them nonblocking. However, alternate paths exist to makeconnections, and not all of them will be nonblocking.

An architecture discussed in the literature that is strict sensenonblocking (no alternative paths that lead to blocking) is the Spanke(SP) architecture. This can be achieved using a combination of existing1×2 and 2×1 switches (e.g., optical fiber directional couplers) inoptical communications networks. The key to the Spanke architecture isthat each input can be independently coupled to N outputs. A Spankearchitecture can be achieved using switchable PDLC transmission gratingsas follows.

Three different types of switchable up/down deflector/acceptorholograms, as illustrated in FIGS. 38( a)–(b) may be utilized inconnector components. Note that in the powered state, all holograms passlight straight through. We designate each hologram by an integer m,where this is the multiple of an angle θ through which the beam isdeflected down as shown in FIG. 38( a). Hence, a hologram designated 2deflects the beam down by 2θ, for example. Holograms designated with abar over the number m deflect light up by mθ as shown in FIG. 38( b).

In an embodiment of the present invention, these types of holograms maybe stacked up in two layers in the configuration shown in FIG. 39 toconstruct a 4×4 Spanke switch. The dashed lines show potential paths formaking connections of any input to any output. At any active input, atmost only one hologram is activated (unpowered). Powering all threeholograms makes a straight line connection. Utilizing thisconfiguration, any of the N possible outputs can be connected to anyinput. In the output, the corresponding acceptor hologram is activatedto make the connection. For example, to make the actual connection frominput 1 to output 3, shown by the heavy line in FIG. 39 hologram 2 isactivated in channel 1 of the input layer, and hologram 2 in channel 3of the output layer is activated. This type of configuration can begeneralized to any N×N switch. This manifestation of the Spankearchitecture requires N−1 different types of holograms (i.e., N−1different deflection angles).

In a further embodiment of the present invention, a Spanke architecturecan also be realized using just one type of switchable transmissionhologram. Consider the symmetric elementary holographic switchillustrated in FIG. 40 which is used to form a 4×4 Spanke switch asshown in FIG. 41. The dashed lines again show potential paths forconnections, while the heavy line shows an actual connection from input3 to output 1. In this configuration, the nodes A, B, C, D, E are staticoptical elements (e.g., conventional mirrors, or holograms, or possiblyoptical fibers) or combinations of such elements that connect the inputand output layers. In other words, they are always activated, but willnot direct optical signals unless a signal is placed in that path by theelementary holographic switches. This can also be generalized to an N×Nswitch for any N. Any Spanke switch will require 2N(N−1) elementaryswitches, which is larger than the number for a corresponding SU switch.

These manifestations of the Spanke architecture can also be generalizedto N parallel columns of up or down deflectors coupled to N parallelrows of right or left deflectors to form an N²×N² matrix cross-connectswitch. A polarization diversity scheme similar to that shown in FIG. 36could then be employed. Finally, it should be noted that all of thesematrix switch concepts can be generalized to NM×NM matrixcross-connects, where N≠M.

In another embodiment of the present invention, a 4×4 (or 2²×2²)cross-connect component 80, is shown in FIG. 42. The firstdeflector/acceptor layers 82 perform the up-down switching, while thesecond set of layers accomplishes right-left switching 84. For example,switching an input from the first quadrant to an output in the fourthquadrant is accomplished with all four PDLC switchable holograms “on.”On the other hand, making a “4-to-3” switch requires turning theholograms in the first and second layers “off,” and leaving theholograms “on” in the third and fourth layers. Any of the four inputscan be mapped to any of the four outputs with this configuration.

In another embodiment of the present invention, a 9×9 (3²×3²)cross-connect switch 90, is shown in FIG. 43. In order to achieve thesame range of deflection movement as the 4×4 matrix, a sub-layer of PDLCswitches 91 is introduced for some middle rows/columns of the principalPDLC layers 92. Additionally, the principal PDLC layers 92 also contain“holes” 93 in some of the middle columns. The sub-layers of PDLCswitches 91 and the holes 93 are necessary to allow for the beams to goup or down and right or left. This cross-connect switch allows thevoltage-select PDLC switches to direct any input to any output. Noticein this case, for example, to direct the top row to the bottom row, theholograms in the second layer will be “off.”

In another embodiment of the present invention, a 16×16 (4²×4²)cross-connect switch 100, is shown in FIG. 44. There are once moresub-layers 91 and holes 93 for the middle rows/columns, interspersedbetween the principal layers of PDLC holographic switches 92. Thisdevice consists of 8 principal layers, 4 for up-down switching, and 4for right-left switching. Any one of 16 inputs can be mapped to any oneof 16 outputs by appropriate voltage-selection of holograms. In general,the progression of devices illustrated in FIG. 42 through 44 shows thata N²×N² cross-connect will require 2N principal layers with 2(N−1)sub-layers. If the vertical/horizontal distance between beams is d, andthe deflection angle is θ, then the total thickness of the device isgiven by

$t = {\frac{( {{2N} - 1} )d}{\tan\;\theta}.}$For example, assuming d=3 mm and θ=20°, the 16×16 cross-connect wouldhave a total thickness t≠58 mm (<3 inches).

The cross-connect switches described herein are fully operational in afree space configuration (though not limited thereto). The principaloptical concerns and limitations of cross-connect switches are insertionloss and cross talk. Ultimately, both of these limitations depend on thediffraction efficiency η of the PDLC holograms. In any of the N²×N²cross-connects, we can readily see that the maximum number ofdeflections will be four (one up/down deflector/acceptor pair and oneright/left deflector/acceptor pair). Hence the minimum throughput of thedesired signal is η⁴. (When only two deflections are required, this willincrease to η².)

By way of example, transmission holograms with η=80% results in aminimum throughput of 41%, or an insertion loss of 3.9 dB. This is theminimum diffraction efficiency resulting from the PDLC holographicswitches. This surpasses currently available commercial devices.Continuing with the present example, PDLC holograms have an indexmodulation of ˜0.024. At a wavelength of 670 nm, this corresponds to acoupling coefficient of κ˜0.11 μm⁻¹. Consequently, for a 10 μm thickhologram η=80% and this diffraction efficiency increases to 95% and 99%for a thickness of 12 μm and 15 μm, respectively. With η=99%, thethroughput is 96%, and the insertion loss is 0.17 dB. This analysisignores losses in the ITO layers, which is minimal.

The analysis of cross talk is more complicated. A first-order analysiscan be accomplished by examining the 4×4 switch of FIG. 42. Note thatthe undiffracted part of the beam passing through the first “down”deflector passes directly to an “up” acceptor. However, in this case theacceptor acts as an “up” deflector since the input is at normalincidence. Thus, this remainder beam is diffracted out of the system(and may be baffled), with a smaller remainder transmitted to the thirdlayer which contains a “right” deflector. Here, it is diffracted again.The possible outputs for a single given input are illustrated in FIG.45. This system acts as a passive filter, attenuating the cross talk. Inthe worst case, with η=80%, the isolation for the desired channel is 9.0dB, and the maximum cross talk in an adjacent channel is down by −12 dB.However, if η=99%, the worst case isolation is 37 dB, and the maximumcross talk in an adjacent channel is down by −40 dB. This analysis doesnot include residual diffraction in a switched hologram since the worstcase considered all of the holograms “on.” However, the diffractionefficiency of a hologram in the “off” state can be <1%, so this shouldnot make the present results any worse. A more detailed analysis shouldinclude minimal ITO losses.

Electrical requirements are also a consideration and possible limitationto the overall efficiency and usefulness of optical cross-connectswitches. For example, given η=80% it is possible to make transmissiongratings that switch at about 5 V/μm. Similarly, 15 μm thick film wouldthus require 75 V for switching.

Power requirements are computed with the following reasoning. The ACpower is given by

${P = {\frac{1}{2}{fCV}^{2}}},$where f is the frequency of the square-wave voltage, C is the filmcapacitance, and V is the applied voltage. The physical size of a N²matrix is Nd×Nd; e.g., for a 4² matrix with d=3 mm, this size is 12×12mm². A typical capacitance for a hologram of this area is ˜2 nF. Anoptimum frequency for high switching contrast is 2 kHz. Thus, with aswitching voltage of 75 V, the power required for one hologram matrix is˜11 mW. For a 4²×4² cross connect with 8 layers and 6 sub-layers (witheach sub-layer requiring half the power of a layer), the net requirementis 11×11 mW=121 mW. If visual desires drive up the size of the devicefor ease of demonstration, note that the net power requirement willscale with total area. Should the power become prohibitive, it ispossible that current-limiting resistors could be used to minimize thepower, since most of the current is associated with thecapacitor-charging spike. Given that the RC time constant is on theorder of a few micorseconds, the demonstration would not be hampered ifthis were stretched out considerably, thereby reducing the AC powerrequired.

While preferred embodiments of the present invention have been describedherein, the disclosure is not intended to be limiting. The presentinvention encompasses any and all modifications, adaptations, andembodiments that would be understood by those in the art based on thisdiscloure.

1. A polarization diversity system comprising: a first polarizing beamsplitter for receiving an input beam of light and splitting the inputbeam of light into a first beam of light polarized in first directionand a second beam of light polarized in a second direction; a firstoptical path comprising: a first deflector made from polymer-dispersedliquid crystal elements; a half-wave plate; a second deflector made frompolymer-dispersed liquid crystal elements; and a mirror, wherein thefirst optical path receives the first beam of light polarized in a firstdirection from the first polarizing beam splitter and outputs the firstbeam of light polarized in a second direction; a second optical pathcomprising: a mirror; a third deflector made from polymer-dispersedliquid crystal elements; a half-wave plate; and a fourth deflector madefrom polymer-dispersed liquid crystal elements, wherein the secondoptical path receives the second beam of light polarized in a seconddirection from the first polarizing beam splitter and outputs the secondbeam of light polarized in a first direction; and a second polarizingbeam splitter for receiving the outputted first beam of light polarizedin a second direction from the first optical path and the outputtedsecond beam of light polarized in a first direction from the secondoptical path.
 2. The polarization diversity system according to claim 1,wherein the first and fourth deflectors are capable of deflecting lightin a first and second direction.
 3. The polarization diversity systemaccording to claim 2, wherein the second and third deflectors arecapable of deflecting light in a third and fourth direction.
 4. Thepolarization diversity system according to claim 1, wherein the firstand second optical paths are symmetrical in length.